Propylene-based elastomers are disclosed in the art. For some end-use applications, such as injection molded articles, thermoformed articles, sheet/film and profile extruded articles these elastomers may exhibit advantageous characteristics versus ethylene-based elastomers. The propylene-based elastomer may be utilized in an end-use application which contains another propylene-based polymer (such as homopolymer polypropylene (hPP), random copolymer polypropylene (RCPP) and/or impact copolymer (ICP) (see, for example, EP0716121A1).
Elastomeric materials tend to have lower crystallinity (typically, heats of fusion between about 1 and 35 J/gram for propylene-based elastomers). These elastomers tend to be softer than typical propylene-based polymers. As a result, the elastomers can stick together at various stages beginning in manufacture (i.e. blocking during and immediately after manufacture), and during various stages of storage and transport, and use by polymer converters.
The tendency for elastomers to stick together can be especially difficult where the elastomer is formed into pellets during manufacture. It is typical for elastomers to stick together soon after formation into pellets before the elastomer is completely cooled. Because propylene-based polymers crystallize significantly more slowly than ethylene-based polymers of equivalent density, they can take longer to develop their full crystallinity. As a consequence, stickiness is particularly problematic for propylene-based elastomers. This characteristic of propylene-based elastomers can create problems and blockages during transport through the polymer manufacturing facility, thereby reducing the rate at which the elastomer is manufactured.
Even after the pellets have been produced in the manufacturing facility with minimal blocking, additional blocking can develop over time at temperature and pressure during storage and transport. Blocking of this type typically occurs as a result of pressure being exerted upon the pellets at various temperatures in containers such as boxes, bags, rail cars, trucks, or silos. This phenomenon can be described as the delayed onset of blocking and is a common problem with lower crystallinity elastomers. Though not intended to be limited by theory, it is thought that thermodynamics drives phase separation at use temperature (i.e. 0 to 65.6° C.) for propylene-based elastomers. Amorphous species (lower molecular weight or otherwise) can migrate (“bloom”) to the surface. It is believed that the homogeneous ethylene-alpha olefin interpolymer component of this invention can minimize this blooming of such amorphous species and also nucleate propylene segment crystallinity, thereby minimizing blocking of the elastomer during the manufacture, shipping, and storage of the elastomer to end-use locations/facilities.
The pellets are conveyed from the polymer manufacturer to the end-use converter where they typically are transferred to holding containers and the like and/or directly to converting equipment. During transport, storage, and use, the pellets preferably should be readily separable from one another with a minimum amount of effort. In the worst case scenario, conversion of the elastomer pellets into end-use articles may be slowed, interrupted, or halted altogether if the pellets form large blocks within the process stream of the article fabrication equipment.
Various additive strategies (i.e. addition of additives, such as slip agents, anti-blocks, oils etc.) and mechanical strategies (i.e. agitation and break-up by means of additional equipment) have been attempted to compensate for the stickiness of elastomeric polymers. However, these so-called remedies often entail significant penalties such as additional costs and may also adversely affect the key properties of the elastomeric polymer. For example, the anti-block additives and modifiers may significantly compromise the optical properties of the propylene-based elastomer and/or the soft flexible behavior of the propylene-based elastomer materials. Homopolymer polypropylenes can be added to the propylene-based elastomer during manufacture, however, such polypropylene will increase the stiffness of the composition and decrease the elastic recovery, which may be unacceptable for some end-use applications.
Elastomeric compositions consisting of an in-reactor blend of a propylene-rich elastomer and an elastomeric polymer that is either non-crystalline or has very low crystallinity and ethylene derived crystallinity are proposed in WO 2006/044149A1. However, such non-crystalline or slowly crystallizing materials, such as low crystallinity elastomers will not significantly improve the stickiness and/or blocking behavior of an elastomeric blend, which includes slow crystallizing propylene-rich elastomers. Additionally, it is believed that the inclusion of such non-crystalline or low crystalline ethylene derived elastomers in a blend which further includes a polypropylene in addition to the propylene-based elastomers will result in unacceptable high haze and low clarity values for certain end-use applications. Furthermore, in some applications, the stiffness of the overall composition will be inadequate.
It would be desirable to have polyolefin elastomeric compositions, which contain a propylene-based elastomer and exhibit excellent physical properties, such as flexibility, while at the same time exhibiting low stickiness and blocking behavior in order to enable the compositions to be effectively and readily manufactured without manufacturing upsets due to blocking of the polymer handling facilities of the manufacturing plant. It would also be desirable for such compositions to be resistant to the delayed onset of blocking during shipping and storage. Preferably, such elastomeric compositions will be compatible with both propylene-based polymers and ethylene-based polymers. Finally, it would be beneficial for such compositions to exhibit excellent optical properties when blended with ethylene-based and propylene-based polymers.